Being very close to 30 and having worked after my MA, the only advice/experience I can share is that work can cause anxiety too. You'll most likely have deadlines or responsibilities/things to do or hand in in whatever field you choose to pursue. Anxiety is something that needs to be tackled on its own, and in my experience getting a bit older helps, as you build up different life experiences and realize some things that seemed super important were actually relatively chill/irrevelant and so on. You might not be attracted to the academic life, and that's a different story. But if you are, and your only worry is stamina, then you might encounter this issue in other fields as well and it would be too bad to miss a great PhD opportunity.
That being said, you do you, and trust your gut, this is just my own standpoint on these matters and maybe we're very different people.